1. Field of the Invention
The present invention generally relates to oscillations circuits and more particularly to a digitally controlled oscillator that provides an immediate recovery from a low-power sleep mode, without any delay.
2. Description of the Related Art
Oscillators are used in many current electronic devices such as portable communication devices (cell phones) and computers. In addition, integrated circuit chips with different modes need to be able to power down the non-active areas to keep operating power low. It is desirable to reduce the power consumed by these devices so as to make the power supply sources smaller and longer lasting.
One source of power consumption within such devices is the oscillators. Therefore, it is desirable to reduce or eliminate the power consumed by the oscillators, when the oscillating signal is not required. However, conventional oscillators generally require a start-up period to produce a stable signal. Therefore, it is common in conventional circuits to allow the oscillators to continue to run in order to avoid the time penalty associated with the start up period.
The start-up time of an oscillator is defined as the time required for the oscillator to reach a steady state. Presently, for most oscillators, the start-up time can be a few milliseconds to several seconds depending on the crystal frequency and amplifier design of the oscillator. The start-up time may be even longer when the temperature of the device using the oscillator increases. The reason for the delay is that when the conventional oscillator circuit is powered up, the output of the amplifying inverter begins to bias the input through a bias resistor. The bias resistor and the load capacitors are large and the amplifying inverter may be weak. This causes considerable delay for the oscillator circuit to reach appropriate bias levels. After start-up, the oscillator circuit losses cause the oscillator circuit to stabilize (i.e., loop gain is approximately one).
One example of a conventional oscillator is found in U.S. Pat. No. 5,834,982, incorporated herein by reference. Such a conventional oscillator, shown in FIG. 1, is a Colpitts-type crystal oscillator. The Colpitts-type crystal oscillator is a Barkhausen-type oscillator having capacitive reactances between the collector and the emitter and between the base and the emitter, respectively, and an inductive reactance between the collector and the base. The Colpitts-type crystal oscillator uses a crystal resonator as the inductive reactance device.
Referring to FIG. 1, the conventional Colpitts-type oscillator includes an oscillating transistor Q1, a capacitor C1, a capacitor C2, a capacitor C3, a crystal resonator X1, and four resistors R1-R4. The capacitor C1, functions as the capacitive reactance between the collector and the emitter. The capacitor C2 functions as the capacitive reactance between the base and the emitter. The series of the crystal resonator X1 and the capacitor C3 has a positive reactance and functions as the inductive reactance between the collector and the base. The two resistors R1 and R2 divide the power source voltage VCC. A connection point of the two resistors R1 and R2 is connected to the base of the oscillating transistor Q1. As a result, a base current IB1 is determined by resistance values of the two resistors R1 and R2 and flows into the base of the oscillating transistor Q1. The collector of the oscillating transistor Q1 is supplied with a collector current IC1, determined by a resistance value of the resistor R4.
This kind of oscillator requires a long a starting time until the oscillator reaches a steady state after the power source is supplied. In the conventional Colpitts-type crystal oscillator, the power source voltage VCC, a grounded-emitter current amplification factor xcex2 of the oscillating transistor Q1, and the collector current IC1 are given by VCC=3V, xcex2.=180, iC1=0.3 mA, respectively. FIG. 2 illustrates the above-mentioned conditions.
In FIG. 2, the starting time TS is defined as a time period required for an output (AC voltage) level VOSC of the oscillating circuit Q1 to reach 90% of an output level Vconst of the oscillator at the steady state after the power source is supplied to the oscillator. As understood from FIG. 2, it is apparent that the starting time TS of this conventional oscillator is about 5.5 msec. Conventionally, the collector current IC1 of the oscillating transistor Q1. is increased in order to reduce the starting time TS. However, if the collector current IC1 of the oscillating transistor Q1, is large (to improve the starting characteristic of the oscillator) the power consumption of the oscillator is undesirably increased.
Therefore, there is a need for a new type of oscillator circuit that does not incur the power or time penalties that are seen in conventional oscillator devices. The invention described below allows the oscillator to be shut off to reduce or eliminate power consumption, yet allows the oscillator to immediately turn on when required, without delay, and without excessive power consumption.
In view of the foregoing and other problems, disadvantages, and drawbacks of conventional oscillator circuits, the present invention has been devised, and it is an object of the present invention to provide an improved oscillation circuit.
In order to attain the object(s) suggested above, there is provided, according to one aspect of the invention a digitally controlled oscillator that includes a state circuit for maintaining the state of the oscillator prior to shutting the oscillator off and method for restoring the oscillator to the saved state when the oscillator is turned on.
The digitally controlled oscillator also includes an adjustable frequency loop for producing an oscillation signal. The state circuit maintains a state of the adjustable frequency loop prior to shutting the oscillator off. The xe2x80x9cstatexe2x80x9d of the adjustable frequency loop is the frequency of the oscillation signal. The invention may also include integration and control logic adapted to control the state circuit to save the state of the oscillator upon receipt of a hold signal. Otherwise, the integration and control logic maintains a frequency of the oscillation signal output by the oscillator when the hold signal is absent. The digitally controlled oscillator also preferably includes an error loop that maintains a frequency of the oscillation signal output by the oscillator within a predetermined range. Upon receipt of an enable signal, the restore circuit causes the oscillator to immediately output an oscillation signal based on the saved state.
In another embodiment, the invention comprises a digitally controlled oscillator that includes an adjustable signal generating circuit adapted to generate an oscillation signal. A feedback loop receives the oscillation signal from the adjustable signal generating circuit. The feedback loop detects error in the oscillation signal and produces an error signal based on the error. The control logic circuit receives the error signal from the feedback loop and maintains the oscillation signal within a predetermined error range. Also, a state device that is connected to the adjustable signal generating circuit maintains a previous operating state of the adjustable signal generating circuit when the digitally controlled oscillator is temporarily powered down.
When the digitally controlled oscillator is powered up after being temporarily powered down, the control logic starts the adjustable signal generating circuit at the previous operating state based upon data maintained within the state device. The xe2x80x9cprevious operating statexe2x80x9d includes the previous frequency of the oscillation signal. Upon being powered up, the digitally controlled oscillator immediately outputs the oscillation signal based upon the previous operating state maintained within the state device, without initially processing the oscillation signal through the feedback loop. However, after the digitally controlled oscillator is powered up and is back in normal operation, the oscillation signal is processed through the feedback loop to maintain the frequency of the oscillation signal within the predetermined frequency range.
An important feature of the invention is that the adjustable signal generating circuit and the feedback loop do not consume power when the oscillator is temporarily powered down.